The following documents are incorporated herein by reference:                IEEE STD 802.15.4; and        IEEE STD 802.15.4e.        
FIG. 1 diagrammatically illustrates a simplified topology of a typical conventional wireless communication network operating in compliance with IEEE 802.15.4e, which is an amendment to the IEEE STD 802.15.4 radio communication protocol specification. This amendment specifically targets MAC protocol level modifications to enhance the performance of devices using IEEE 802.15.4 radios. A specific IEEE 802.15.4e MAC operation is referred to as Time Slotted Channel Hopping (TSCH), which enables robust as well as low-power communication. Channel hopping provides robustness against interference. The time slotted and time-synchronized nature of the protocol allows for time-scheduled communication. The scheduled communication means that the devices need to be active only when required and in “sleep” mode otherwise. This provides low-power operation.
As shown, the topology of FIG. 1 has a hierarchical structure with a plurality of hierarchical levels. The network of FIG. 1 includes at 11 a central root node (RN) that controls the network and serves as a gateway to higher bandwidth networks. A Level 1 intermediate node (IN) at 12 interfaces between the RN and a plurality of Level 2 INs at 13, which each in turn interfaces between the Level 1 IN 12 and one or more leaf nodes (LNs) at 14. The wireless communication links illustrated between the various nodes of FIG. 1 are also referred to herein as communication hops (or simply hops), such that, for example, RN 11 is three hops away from each LN at 14.
TSCH uses a sequence of time frames that are each divided into a plurality of slots of time (time slots). In the IEEE 892.15.4e scheduling scheme, each node is allocated a shared slot and at least one beacon slot in each time frame. A beacon slot is used by the RN 11 to transmit a beacon packet that contains the transmit/receive schedules of nodes in the network. Beacon slots are also used for time synchronization. The shared slot is a shared transit/receive slot (STRS) in which a node may either transmit or receive. New nodes attempting to associate with (join) the network contend for access to the STRS, and a successfully contending node transmits a network association request in the STRS. The STRS is the same interval of absolute time for each node of FIG. 1. The STRS is also used to convey network maintenance information and traffic routing information.
The RN 11 uses the STRS to transmit an association response in reply to an association request received from a successfully contending node. As new nodes join the network, they receive respectively corresponding association responses that allocate to them, in sequential fashion, respective pairs of dedicated slots. Each pair of dedicated slots consists of a transmit slot and a receive slot, both reserved for use only by the node to which they are allocated. If the joining node is an IN, the association response also allocates an additional beacon slot to the IN, to support the relay of beacon packets across the hops from the RN to the LNs.
A node operating in the network wakes up from sleep mode at every time slot and checks what function is to be performed. If none, it returns to the sleep state. The node listens in an active receive state during a beacon slot, an STRS and a dedicated receive slot, in order to ensure that no communication is missed. In a dedicated transmit slot, the node transmits as needed, or returns to sleep if it has nothing to transmit.